Solid laser laterally pumped with focussed light from diodes in multiple flows

ABSTRACT

The invention relates to a solid laser, wherein a laser active material is pumped with the aid of at least one pump light source, e.g. of one of several laser diode arrays, at least in an approximately perpendicular manner in relation to the axis of a laser beam extending essentially inside said laser material. The pump beams are reproduced or focussed in said material with the aid of focusing optical elements, e.g. cylindrical lenses. At least one boundary surface, which is arranged opposite the incident surface, is provided in the material and is embodied in such a manner that the pump beams are reflected thereon and radiate once more through the laser material and/or such that an external reflector is arranged behind said opposite boundary surface and returns the pump beams into the material. The laser material can be doped in partial areas only.

Contrary to end-pumped rod lasers, with transverse or side-pumped lasersit is significantly more difficult to overlap pump light and laser modeoptimally. In conventional systems the pump light coming from one ormore pump light sources, e.g. diode arrays is either side beameddirectly into a cylindrical crystal or focussed into the axis region ofthe crystal. In such assemblies the slow axis of the diode array isusually oriented parallel to the crystal axis. Since, after entering thecrystal, the intensity of the pump light is diminished exponentially dueto absorption, resulting in a considerable proportion of the pump powerbeing aborbed in the direct vicinity of the entry location, wheras thelaser mode forms in most assemblies along the crystal axis so that thedistance between entry location and laser mode is at least of the orderof 1 mm with crystal diameters as currently achievable, the overlap isweak and thus the efficiency of the laser low. Morover, because of theabsorbed pump power being distributed unsymmetrically there is theproblem that higher order transversal modes are excited to the detrimentof beam quality.

Since, however, the latest pump light sources of interest, namely laserdiode arrays having exceedingly slim elongated emitting surfaces makeside-pumped systems superior in principle to end-pumped lasers, becauseof eliminating the expense of shaping the pump beam, it is the object ofthe present invention to create a substantial improvement by providing alaser of high beam quality making more efficient use of the incidentpump power. This is now possible with the aid of an assembly as it readsfrom claim 1 of the present invention by surprisingly simple ways andmeans. Advantageous further embodiments of the invention read from thesubject matter of the sub-claims.

In accordance with the invention the pump beam coming from a lightsource e.g. a laser diode array is beamed roughly perpendicular to thelaser beam axis, but preferably slightly angled to the perpendicular ofthe surface of a laser material to enter into the latter. To ensureefficient utilization of the pump power the pump beam is focussed byoptical elements such as e.g. lenses or mirrors on the laser material oran image of the emitting pump light surface area generated whose widthis set to achieve good overlap of the pumped region and the laser beam.Since the side of the laser material facing away from the pump lightsource is preferably coated reflective, the pump light beam is reflectedat the side of the laser material facing away from the pump light sourceto again pass therethru, resulting in a greater proportion of the pumppower being absorbed. As an alternative or in addition to coating thelaser material, a reflector, e.g. a mirror can be provided behind thefacing away side of the laser material by which which the pump beam isreflected back into the material. The efficiency can be further enhancedconsiderably by returning or imaging the pump beam after having emergedfrom the laser material back into the material by a reflector, e.g. amirror and then again reflected to the rear wall of the laser material,whereby the size and position of this second image is preferablyselected to achieve good overlap with the laser beam. This why thesecond image is expediently located directly juxtaposed to the firstimage or coinciding therewith. It is in this way that the pump lightbeam is directed four times through the same pumped region of the lasermaterial, resulting in highly effective utilization of the pump power.In an alternative version the second image of the pump beam is locatedat a certain distance away from the first and the pump beam, afterhaving passed through a second region, is reflected back by the mirrorinto the first region. In this case it is expedient when a second pumpbeam passes through the two regions in another sequence. Where very weakabsorbing materials are involved it may prove expedient to direct thepump beam in a similar way through two or more regions and also todirect the laser beam with the aid of diversion mirrors through all ofthese regions.

It is to be noted that when such indications in this patent asperpendicular or parallel are rendered relative by “approximately” or“substantial”, this means that the direction is mainly as indicated butthat departures of e.g. 20 deg are just as possible.

The pump light source is configured preferably elongated, in other wordsextending significantly longer in one direction than in the other, orconsisting of a train of small pump light sources along a preferreddirection. Extending roughly parallel to the latter is also the pumpedregion along a preferred direction. The laser beam, which may also befolded, passes through the pumped regions preferably along thispreferred direction in thus extending substantially between the surfacesof the laser material facing and facing away from the pump light sourceand thus also approximately parallel to the pump light source.

The laser material of the invention may be of any geometry adapted tothe particular purpose, e.g. in the shape of a rod or slab. In thesimplest case the material is slab-shaped, although it is proposed toachieve higher laser powers to employ rods of square or hexagonalcross-section, etc.

Cooling the laser material can be done both with the aid of a fluid flowand with the aid of a solid-state material of high thermal conductivity.Where a fluid flow is employed it is proposed to allow it to flow overthe surfaces of the laser material facing and facing away from the pumplight source and to dimension temperatures and/or cross-sections of theflow passages so that a symmetrical distribution of temperature and thusa symmetrical thermal lens materializes as best possible in the lasermaterial through which the laser beam is directed.

Using polarization and beam splitter elements as a function of thepolarization achieves not reflecting the pump light beam back in thedirection of the pump light source after the fourth passage through thelaser material, it instead impinging a further reflector by which thebeam is again returned back into the laser material. This is achieved byrotating the beam on its way in its direction of propagation, e.g. bylambda quarter slabs. Provided in front of the pump light source in thiscase is a polarization beam splitter which ensures that pump beam comingback from the laser material and rotated in the polarization plane takesa path different to that originally, i.e. it no longer being returned tothe pump light source, but instead directed at a reflector by which itis in turn directed into the pumped region of the laser material. It isin this way that it is now possible to pass the pump beam through thepumped region eight times in all, as explained in more detail withreference to FIG. 2.

Instead of for a laser the invention can be employed just as well for alaser amplifier, it being proposed in this case that the side surfaceareas of the laser material are coated antireflective for possible laserwavelengths to prevent parasitic transversal modes materializing whichwould rob the pumped region of beam power. This can also be prevented asan alternative by lightly slanting opposite surfaces of the lasermaterial and/or roughening the side surface areas.

Technical elements of the laser such as e.g. pump light sources, laserbeams and optical elements may be provided singly or in a plurality.Where two linear pump light sources are used, these are arranged in lineand/or staggered at an angle of preferably 90 deg.

Although the laser beam is aborbed in the laser material it may,however, emerge therefrom at the end faces located in the preferreddirection of the pumped regions.

In one advantageous further embodiment of the invention as it reads fromthe main patent it is proposed to use a laser material which is dopedonly in internal regions. One preferred version thereof is shown in theFIG. in which the laser material takes the form of a slab made up ofthree layers, of which only the middle layer (25) is doped, whilst thetop and bottom layers (24) are undoped. This results in the pump beambeing aborbed only in the doped layer in thus achieving a better overlapbetween the pumped region and the laser mode. Propagation of thedecaying portion of the laser mode, the evanescent wave, in the undopedregions is practically lossless.

The invention will now be detailled, for example, by way of preferredexample embodiments with reference to the drawing in which:

FIG. 1 is a section through an assembly in accordance with the inventiontransverse to the linear extent of the laser rod 1, configured as a slabon which a laser diode array 5 is imaged from above,

FIG. 2 is a section through an assembly in accordance with the inventiontransverse to the linear extent of the laser rod 1, configured as a slabon which a laser diode array 5 is imaged from above, whereby with theaid of a lambda quarter plate 11 and a polarization beam splitter theincident and back-reflected pump beam are separated so that the pumpbeam is directed through the slab eight times with the aid of mirrors,

FIG. 3 is a section through an assembly in accordance with the inventiontransverse to the linear extent of the laser rod 1, configured as a slabon which two laser diodes 5 are imaged from above,

FIG. 4 is a section through an assembly as an alternative to that asshown in FIG. 3 on which laser diodes 5 and focussing lenses 13, on theone hand, and reflection mirrors 7, on the other, are arrangedalternatingly as regards the perpendicular on the laser slab 1,

FIG. 5 is a section through an assembly in accordance with the inventiontransverse to the linear extent of the laser rod 1, configured as a slabon which unlike in FIG. 1 the fluid cooling is replaced by heat sinks ofsolid-state material.

FIG. 6 is a section through an assembly in accordance with the inventionin which the laser slab is additionally pumped from the left with theaid of a laser diode 5,

FIG. 7 is a section through a laser resonator in accordance with theinvention including a laser rod 1 which is pumped from above by a pumpdiode 5.

FIG. 8 is a section through a laser resonator in accordance with theinvention including a laser rod which is pumped from above with two pumpdiodes arranged in line with the slow axis, and

FIG. 9 is a section through a laser resonator in accordance with theinvention in which the laser rods 1 are arranged in a zig-zagconfiguration,

FIG. 10 is a section through an assembly in accordance with theinvention in which the pump beam, after having left the laser slab fromthe top is imaged into a second region of the slab not coinciding withthe first, from which it is reflected back and directed back into thefirst by mirrors,

FIG. 11 is a section through a laser resonator in accordance with theinvention transverse to the pump beams wherein the laser beam isdirected by diversion mirrors through the two pumped regions as shown inFIG. 10.

Parts which are like, or like in function, are identified by likereference numerals in the FIGs.

Referring now to FIG. 1 there is illustrated unlike conventionalassemblies a thin slab 1 of laser gain material between two plates 2 and3 of glass or some other material transparent to the pump radiation. Theinterspaces between laser slab and glass plates are filled with a fluidcoolant 4 which is likewise transparent to the pump radiation. Theunderside of the laser slab 1 is coated highly reflective for the pumpradiation, whilst the upper side is coated antireflective. The pump beamcoming from the diode array 5 is imaged with the aid of a cylindricallens 6 curved in the direction of the fast axis of the diode array,through the upper glass slab and the coolant on the underside of thelaser slab in a relatively narrow strip, the width of which is specifiedmore accurately below as a function of other parameters. The angle ofincidence formed by the axis of the pump beam to the normal on the pumplight, is preferably roughly of the magnitude of half the aperture angleof the beam, but may also be larger or smaller than this. The reopeningbeam reflected at the underside of the laser slab impinges thecylindrically curved collimating lens 7 which re-images the beam on theunderside of the pump light, the radius of curvature of the mirror beingselected so that the second image of the beam is roughly the same inmagnitude as the first and overlapping it. The beam is then againreflected at the underside of the laser slab in the direction of thecylindrical lens 6. Since the laser slab is passed through four times inthis way, this assures that a considerable proportion of the pumpradiation is absorbed in the slab, resulting in an occupany inversionbeing built up in the beamed region of the laser slab. The laser rod 8is oriented roughly through the middle of the pumped regionperpendicular to the imaging plane.

To further boost the efficiency of the pump assembly as described above,it is proposed to polarise the radiation of the laser diode. Referringnow to FIG. 2 there is illustrated a corresponding assembly inaccordance with the invention as will now be described. Unlike theassembly as shown in FIG. 1, in this version a polarization beamsplitter 9, known as such, is inserted between the lens 6 and laser slab1. Various embodiments of such beam splitters are available. For theassembly as shown in FIG. 2 e.g. a Foster prism was selected. Thiscomprises two prismatically ground bodies of a strongly birefringentmaterial, e.g. calcite, whose optical axis is oriented perpendicular tothe image plane, resulting in the refractive index assuming a differentvalue for baams polarized in or perpendicular to the imaging plane. Thetwo bodies are joined together along an interface 10. Depending on theversion involved either a narrow air gap between the bodies remains orthe gap is filled with an optical cement whose refractive index issignificantly smaller than the refractive index of the birefringentmaterial. The polarized beam (• • •) coming from the laser diode 5,perpendicular to the image plane is in turn rendered convergent with theaid of a cylindrical lens, enters the Foster prism 9 from above on theright at a slanting angle. The angle at which the beam impinges theinterface 10 is selected so that it is greater than the interface angleof total reflection, resulting in the pump beam being totally reflectedat the interface 10 in then leaving the Foster prism in the direction ofthe laser slab. The refraction of the lens 6 is selected so that thepump beam is imaged, as shown in FIG. 1, as a narrow stripe on theunderside of the laser slab where it is reflected and passes through, onits way to the mirror 7—now different to that as shown in FIG. 1—alambda quarter plate 11 which converts the linear polarized light intocircularly polarized light. Although on reflection at the mirror 7 thesense of rotation of the polarisation relative to the direction ofpropagation is maintained, but because of the latter reversing, theactual sense of rotation of the polarization also changes. Thus,although the beam impinging the lambda quarter plate from above isreconverted back into linear polarized light, but its direction ofpropagation is now rotated through 90° as compared to the originaldirection of propagation. The direction of propagation of the pump beamis thus in the image plane (

□) when the pump beam on its way to the laser slab has passed throughthe lambda quarter plate for a second time. This beam is then directedby the underside of the laser slab into the Foster prism, but, becausethe refractive index of the birefringent material for light polarized inthe image plane is smaller, is no longer totally reflected at theinterface 10, but passes through the latter with no change in directionand only minor losses in intensity. The beam then exits the Foster prismat the upper side is reflected back by the cylindrical mirror 12 intothe Foster prism, imaged at the underside of the laser slab, redirectedto the mirror 7 where it is reflected back to the laser slab. After itsreflection at the mirror 12 the pump beam thus passes through the laserslab another four times, in other words it passing through the laserslab a total of eight times on its full way from the laser diode. Thiswhy in this version, unlike that as shown in FIG. 1, a significantlyhigher proportion of the pump radiation is absorbed in the laser slab,e.g. approximately 80% with Nd-YAG when using currently commerciallyavailable laser diodes and a slab thickness of 0.5 mm. As alreadymentioned above, many different versions are known for polarization beamsplitters which are more or less suitable for the purpose of thisassembly. The gist of this advantageous configuration of the inventionis thus the achievement of additional passes of the pump beam throughthe laser slab with the aid of rotating the polarization plane andmaking use of polarization beam splitters.

Referring now to FIG. 3 there is illustrated how yet a further increasein the absorbed pump power is achieved by imaging the light of aplurality of laser diodes in the laser slab, FIG. 3 showing two laserdiodes, although of course it is just as possible to image more than twodiodes 5 in the laser slab by the principle as disclosed in FIG. 3.Since the aperture angle of the beam leaving the lens 6 is relativelysmall, when the distance between the lens and laser slab is selectedcorrespondingly large, two or more lenses can be arranged juxtaposed, asshown in FIG. 3, for imaging the beams of the diodes on the laser slab.To further diminish the aperture angle of the beams and thus the angleof incidence of the outer beams, it is proposed to use instead of simplecylindrical lenses a system of lenses 12 for the purpose of reducingspherical aberrations. Since such systems of lenses are prior art, theyare depicted in FIG. 3 merely diagrammatically. To further diminish theangle of incidence of the beams on the laser slab—meaning the angleformed by the beams to the perpendicular on the slab 1—it is proposed toadditionally insert a cylindrically divergent lens 14 in the beam pathin front of the upper glass plate 2. Unlike irradiation with just asingle diode, irradiation with a plurality of diodes makes it possibleto control the beam profile as arriving at the laser slab as a whole bydefiningly overlapping the profiles of the individual beams. To achievethis it is proposed to image the individual beams not exactly on eachother but to slightly shift their beam profiles to the left or rightsomewhat to achieve in this way a more box-shaped overall profile tobetter approximate the resulting temperature distribution parabolically.To also achieve an eightfold multipass of the individual pump beamsthrough the laser slab it is proposed, in this case too, to rotate thepolarization planes of the individual pump beams with the aid of e.g.lambda quarter plates to separate the beam paths with the aid ofpolarization beam splitters and to reflect the rotated beams back to thelaser slab with the aid of additional mirrors 12 analogous to theassembly as shown in FIG. 2. This alternative version is not depictedgraphically, however.

Referring now to FIG. 4 there is illustrated how to facilitate thethree-dimensional assembly of the elements and to render the transversalprofile of the pump beam overall more symmetrical it is proposed toarrange laser diodes 5 and focussing lenses 13, on the one hand, andreflection mirrors 7, on the other, alternatingly as regards theperpendicular to the laser slab 1.

Referring now to FIG. 5 there is illustrated an assembly in accordancewith the invention in which, unlike the assembly as shown in FIG. 1,fluid cooling is replaced by solid-state heat sinks of high thermalconductivity which in the form of four plates 17 cool the laser slab topand bottom. The gap between the two upper plates permits entry of thepump beam into the laser slab. To ensure that the resulting temperaturedistribution is symmetrical the two lower plates can be optionallyseparated by a gap.

Referring now to FIG. 6 there is illustrated how in increasing the totalabsorbed pump power still further, as is desirable e.g. for applicationsinvolving material processing, it is proposed to pump the laser rod notonly from above but also from the left or right side or from below, inother words on several sides. In the assembly as shown in FIG. 6 afurther laser diode 5 is imaged with the aid of a lens 6 from the leftin the laser rod 1 whose cross-section is practically square, isreflected at the interface on the right and reflected back into thelaser rod by a cylindrical lens 7 the same as with the irradiationalready described above. The sides of the laser rod 1 are surrounded bya container, housing or case 18 transparent to the pump radiation. Inthe space between laser rod and case there is a flow of coolant 4. Thelaser beam 8 is formed within the rod lengthwise. To further boostperformance it is also proposed in this case to insert polarization beamsplitters and lambda quarter plates in the beam path and/or to irradiatewith a plurality of diodes as already described with reference to FIGS.2 to 4 for irradiation from above. To further increase the laser powerit is proposed to use a laser rod hexagonal or octagonal incross-section and to to irradiate it from correspondingly as many sides.

Designing the optical resonator depends on the properties of the lasermaterial employed. With materials having a positive derivation dn/dT ofthe refractive index n in accordance with the temperature T such as Nd;YAG a more or less symmetrical thermal lens materializes along thepumped region, i.e. perpendicularly to the image plane as shown in FIGS.1 to 6, in which the laser mode is guided as in a waveguide. In thiscase it is sufficient to grind the end faces of the slab orientedperpendicular to the pump region flat, to mirror-finish them and to usethem as end faces for a laser resonator. Depending on the magnitude ofthe differential quotient dn/dT and the absorbed pump power the thermallens effect may be so strong that the transversal profile of the laserbeam becomes too narrow to adequately overlap the pumped region,resulting in a reduction in the efficiency of the laser. To preventthis, it is proposed to coat the flat end faces of the laser slabantireflecting for the laser radiation and to use separate end mirrors15 for the laser resonator, as evident from FIG. 7. To render the designas simple as possible these external mirrors are preferably flat,although curved mirrors may prove advantageous, circumstancespermitting. When a laser material is used in which the derivation of therefractive index according to the temperature disappears or becomesnegative, curved end mirrors of the laser resonator will even benecessary. To control the overlap of pumped region and laser mode, it isfurther proposed as an alternative to optimze the width of the pumpedregion by altering the focal lengths of the lenses 6 and mirrors 7.

Referring now to FIG. 8 there is illustrated an assembly in accordancewith the invention in which two linear pump sources, e.g. laser diodearrays 5 are arranged inline in the direction of the slow axis, whereby,of course, a plurality of diodes could likewise be arranged in the sameway.

Referring now to FIG. 9 there is illustrated an assembly in accordancewith the invention in the form of a folded resonator in which aplurality of laser slabs is arranged in a zig-zag formation betweenmirrors 16. The laser diodes (not shown) whose beams are imaged in thelaser slabs analogous to the assemblies as shown in FIG. 1, 2 or 3 arelocated in front of the image plane perpendicularly above the slabs. Asan alternative it is proposed to replace the individual slabs by a solefull-length slab, but with the laser diodes still in a zig-zagformation.

Referring now to FIG. 10 there is illustrated how to reduce thetechnical complications involved in achieving the assemblies asdescribed above without diminishing the efficiency by an assemblyattaining an eightfold multipass of the pump beam through a laser slab 1without the aid of polarization beam splitters. For this purpose thepump beam emitted by the pump source 5 and imaged by the lens 6 on thelaser material 1 after having left the laser slab 1 at the top, isimaged by a diversion mirror 19 on a region located to the left of theregion first passed through. Here, the pump beam is re-reflected at theunderside of the slab, directed to a mirror 20 which reflects it backinto the second region before then being returned into the first regionby the reflection of the diversion mirror 19. At the same time a secondpump beam from the laser diode 21 is imaged by a lens 6 in the region onthe left, reflected by the underside of the laser slab and, after havingleft the laser slab, is directed at a mirror 22 by which it is imaged inthe pumped region on the right perpendicularly from above. In otherwords, after having been reflected at the underside of slab it isre-directed to the mirror 22 which images it again in the pumped regionon the left. Thus, both pump beams pass through the two pumped regionseight-times each. As readily evident, it is not necessary that thesecond pump beam is imaged perpendicularly on the region to the right.If this is not the case, a further diversion mirror is required. Apartfrom this, it is possible to direct the pump beams with the aid ofdiversion mirrors through three or more juxtaposed regions as long asthis makes sense technically for absorbing the pump beams. Shown in FIG.11 is a resonator in accordance with the invention in which the laserbeam is directed with the aid of diversion mirrors 23 through two pumpedregions.

It is further proposed to make use of the assemblies as shown in FIGS. 1to 11 for pumping and cooling laser rods for laser amplifiers bycoupling an external laser beam from the face end along the pumpedregions into the laser rod. To prevent parasitic transversal laser modesin the rods in this use of the invention, it is proposed to coat theside surfaces of the laser slabs or rods with an antireflective coating,as effective for the laser wavelengths in question, and to grind themnot precisely parallel to each other. If the pump beams come exclusivelyfrom above it is proposed to roughen the right and left-hand sidesurface of the rod.

1-36. (canceled)
 37. A solid-state laser or laser amplifier comprising:a laser gain material for emitting a laser beam along an axis, at leastone pump light source for pumping the laser gain material with a pumpbeam that enters the laser gain material through an entry surface areaalong an axis at least approximately perpendicular to the axis of thelaser beam, and optical elements for focussing the pump beam in saidlaser material, wherein the laser gain material has an entry surfacearea and at least one interface opposite the entry surface area, saidinterface being configured such that said pump beam is reflected by saidinterface to again pass through said laser gain material.
 38. Thesolid-state laser as set forth in claim 37, comprising a reflector whichreceives said pump beam after reflection by said interface and directssaid pump beam back into said laser gain material for a further time.39. The solid-state laser as set forth in claim 38, wherein thereflector is a cylindrical mirror.
 40. The solid-state laser as setforth in claim 38, wherein the optical elements and the reflector createrespective images that overlap partly or fully.
 41. The solid-statelaser as set forth in claim 37, wherein said surface area through whichsaid pump beam enters said laser gain material is flat.
 42. Thesolid-state laser as set forth in claim 41, wherein said interface atwhich said pump beam is reflected is flat.
 43. The solid-state laser asset forth in claim 42, wherein the axis of the laser beam issubstantially parallel to said surface area and said interface.
 44. Thesolid-state laser as set forth in claim 37, wherein said interface atwhich said pump beam is reflected is flat.
 45. The solid-state laser asset forth in claim 37, wherein the pump beam is polarized in a firstpolarization state and the laser further comprises a reflector thatreceives said pump beam after reflection by said interface and directssaid pump beam back into said laser gain material for a further time,and an optical element that is interposed between said laser gainmaterial and said reflector and through which said pump beam passes whenpassing to and from the reflector and which alters the state ofpolarization of the pump beam to a second polarization state.
 46. Thesolid-state laser as set forth in claim 45, wherein the pump beam islinearly polarized and the optical element interposed between said lasergain material and said reflector is a lambda quarter plate.
 47. Thesolid-state laser as set forth in claim 45, further comprising aseparating element located between the pump light source and the opticalelement that alters the polarization state of the pump light, whereinsaid separating element receives both pump light in the firstpolarization state and pump light in the second polarization state anddirects the pump light in the first polarization state to the laser gainmaterial and directs the pump light in the second polarization state toa reflector that reflects the pump light in the second polarizationstate to the laser gain material.
 48. The solid-state laser as set forthin claim 47, wherein the separating element is a polarization beamsplitter.
 49. The solid-state laser as set forth in claim 37, comprisinga plurality of linear pump light sources arranged perpendicularly totheir linear extent and juxtaposed laterally, and wherein the pump lightsources emit pump beams that impinge on said laser gain material atdiverse angles of incidence.
 50. The solid-state laser as set forth inclaim 37, comprising a plurality of linear pump light sources arrangedin line parallel to their linear extent for the purpose of pumping astripe region of the laser gain material, said stripe region being of alength that is a multiple of the length of said individual pump lightsources.
 51. The solid-state laser as set forth in claim 50, wherein theplurality of pump light sources are separated from each other.
 52. Thesolid-state laser as set forth in claim 50, wherein the plurality ofpump light sources are arranged in groups.
 53. The solid-state laser asset forth in claim 50, wherein said stripe region is composed ofdiscrete segments.
 54. The solid-state laser as set forth in claim 37,comprising at least two heat sink members of high thermal conductivityfor cooling the laser gain material, the heat sink members beingseparated from each other by a gap that allows the pump light to enterthe laser gain material.
 55. The solid-state laser as set forth in claim37, wherein the laser gain material is configured as a rod with at leasttwo main surfaces each having an entry surface area, the rod hasinterfaces opposite the entry surface areas respectively, the lasercomprises at least two pump light sources for pumping the laser gainmaterial with respective pump light beams that enter the rod throughsaid entry surfaces respectively, the laser further comprises opticalelements for imaging the pump light sources into the laser gainmaterial, and the interfaces are configured to reflect the pump beamsthat enter the rod through the respective entry surface areas to againpass through the laser gain material.
 56. The solid-state laser as setforth in claim 37, characterized in that some or all of said technicalelements defined in the preceding claims for imaging, redirecting,reflecting or polarizing said pump beams also find application for saidbeams coming from the other side(s).
 57. The solid-state laser as setforth in claim 38, wherein the optical elements create an image in afirst region of the laser gain material, the reflector creates an imagein a second region of the laser gain material, and the laser comprises afurther reflector that receives pump light reflected by the reflector,from the second region, and directs the received pump light to saidfirst region.
 58. The solid-state laser as set forth in claim 57,comprising a diversion reflector that receives pump light that haspassed through said second region and directs the received pump lightinto a third adjacent region, and so on, to then be directed from saidlast region passed through in the reverse sequence through said regionsas passed through prior.
 59. The solid-state laser as set forth in claim57, comprising a second pump light source that emits a second pump beamthat passes through the second region, and a diversion reflector thatreceives the second pump beam from the second region and directs thesecond pump beam to the first region.
 60. A solid-state laser or laseramplifier comprising: a laser material having an entry surface area andat least one interface opposite the entry surface area, at least onepump light source for pumping the laser material with a pump beamthrough said entry surface area along an axis at least approximatelyperpendicularly to the axis of a laser beam substantially absorbed inthe laser material, optical elements for focussing the pump beam in saidlaser material, and an external reflector following said oppositeinterface for receiving said pump beam and reflecting said pump beamback into said laser material, whereby the pump beam again passesthrough the laser material.
 61. The solid-state laser as set forth inclaim 60, wherein the reflector is a cylindrical mirror.
 62. Thesolid-state laser as set forth in claim 60, wherein the optical elementsand the reflector create respective images that overlap partly or fully.63. The solid-state laser as set forth in claim 60, wherein said surfacearea through which said pump beam enters said laser gain material isflat.
 64. The solid-state laser as set forth in claim 63, wherein saidinterface is flat.
 65. The solid-state laser as set forth in claim 64,wherein the axis of the laser beam is substantially parallel to saidsurface area and said interface.
 66. The solid-state laser as set forthin claim 60, wherein said interface is flat.
 67. The solid-state laseras set forth in claim 60, comprising a plurality of linear pump lightsources arranged perpendicularly to their linear extent and juxtaposedlaterally, and wherein the pump light sources emit pump beams thatimpinge on said laser gain material at diverse angles of incidence. 68.The solid-state laser as set forth in claim 60, comprising a pluralityof linear pump light sources arranged in line parallel to their linearextent for the purpose of pumping a stripe region of the laser gainmaterial, said stripe region being of a length that is a multiple of thelength of said individual pump light sources.
 69. The solid-state laseras set forth in claim 68, wherein the plurality of pump light sourcesare separated from each other.
 70. The solid-state laser as set forth inclaim 68, wherein the plurality of pump light sources are arranged ingroups.
 71. The solid-state laser as set forth in claim 60, comprisingat least two heat sink members of high thermal conductivity for coolingthe laser gain material, the heat sink members being separated from eachother by a gap that allows the pump light to enter the laser gainmaterial.
 72. The solid-state laser as set forth in claim 60, whereinthe optical elements create an image in a first region of the laser gainmaterial, the reflector creates an image in a second region of the lasergain material, and the laser comprises a further reflector that receivespump light reflected by the reflector, from the second region, anddirects the received pump light to said first region.
 73. Thesolid-state laser as set forth in claim 72, comprising a diversionreflector that receives pump light that has passed through said secondregion and directs the received pump light into a third adjacent region,and so on, to then be directed from said last region passed through inthe reverse sequence through said regions as passed through prior. 74.The solid-state laser as set forth in claim 72, comprising a second pumplight source that emits a second pump beam that passes through thesecond region, and a diversion reflector that receives the second pumpbeam from the second region and directs the second pump beam to thefirst region.